This invention relates to the production of anhydrous magnesium chloride feed material for use in the electrolytic production of magnesium metal.
More than seventy percent of magnesium is produced electrolytically using magnesium chloride (MgCl2) as feed material. All of the magnesium production processes are fundamentally similar. They are carried out at about 725xc2x0 C. using molten salt electrolytes consisting of magnesium chloride, sodium chloride (NaCl), potassium chloride (KCl), calcium chloride (CaCl2) and a small amount of calcium fluoride (CaF2). They differ only in preparation and use of their respective feed material MgCl2.
Fifty percent cost and energy consumption involved in the current production of magnesium is attributed to magnesium chloride feed preparation. Known current processes of magnesium chloride preparation are cumbersome, complicated and very energy intensive. They are as such because magnesium oxide (MgO) is thermodynamically slightly more stable than magnesium chloride at about 1000K and above, making it difficult to prepare magnesium chloride free of magnesium oxide at these temperatures. Anhydrous magnesium chloride free from magnesium oxide is required as feed material for the good performance of the electrolytic magnesium production cells.
It would thus be desirable to provide a less costly process of preparing anhydrous magnesium chloride suitable for use in the electrolytic magnesium production cells without deteriorating their performance. The present invention provides such a process.
The existing preparation processes may be divided into two categories:
1. low temperature dehydration processes
2. high temperature chlorination processes.
Magnesium chloride hexahydrate is the starting material in the low temperature dehydration processes. It is prepared from sea water or brine through concentration by removing the excess water or even from magnesite by a chemical process. The hexahydrate is further dehydrated by heating in an appropriate manner and atmosphere to avoid hydrolysis reaction to form magnesium hydro-oxychloride and magnesium oxide. The hydrolysis reaction is also avoided by complexing magnesium chloride with potassium chloride, ammonium chloride or ethylene glycol. These processes are multistage and consume large amounts of energy, and some pose health hazard concerns. None of these known processes are believed to be capable of producing low-cost magnesium chloride.
In the second category of high temperature chlorination processes, magnesium oxide is converted into magnesium chloride by reacting with chlorine at about 800xc2x0 C. Magnesium oxide is obtained from magnesite by calcination or from brines by usual methods of preparation. Straight chlorination of magnesium oxide with chlorine gas is thermodynamically not spontaneous, therefore, a reducing agent such as coke with magnesium oxide in some cases and carbon monoxide, methane, or ethane, etc. with chlorine in other cases is used to drive the chlorination reaction forward. The thermodynamic barrier in the chlorination process is thus removed by using a reducing agent, but the chlorination reaction is heterogeneous and this heterogeneity of the chlorination process creates a kinetic problem, making the chlorination processes inefficient. The kinetic problem comes because magnesium oxide is insoluble in the chlorination medium. The medium thus becomes a barrier for MgO to come in contact with other reactants necessary to carry the reaction forward. Attempts at solving this problem by stirring the medium have shown to achieve only partial success.
The present invention overcomes the barrier problem by providing a magnesium oxide chlorination process carried out in a salt medium in which magnesium oxide is soluble and thus able to have sufficient contact with the reactants to convert the MgO.
A method of producing MgCl2 starting material for use in the production of pure magnesium comprises the steps of: a) adding MgO feed material to a starting melt of MgCl2xe2x80x94CeCl3 to yield MgCl2+CeOCl, b) reacting the MgCl2+CeOCl with Cl2 in the presence of a reducing agent to yield a second melt of MgCl2xe2x80x94CeCl3 having a higher MgCl2 concentration than that of the starting melt, c) repeating steps a) and b) with the second melt to further increase the concentration of MgCl2 in the second melt, d) reacting the concentrated second melt of MgCl2xe2x80x94CeCl3 with MgO and Cl2 in the absence of a reducing agent to convert the CeCl3 to CeO2 (s) to yield a melt of pure MgCl2+CeO2 precipitate, and e) separating the CeO2 precipitation from the MgCl2 to yield pure MgCl2.
Thus, according to this simple process, the use of CeCl3 enables the otherwise troublesome MgO to dissolve in the MgCl2xe2x80x94CeCl3 starting melt by readily reacting with the CeCl3 to produce CeOCl and MgCl2, whereupon the CeOCl is converted to insoluble CeO2 from which the MgCl2 can be readily separated to yield pure MgCl2 usable in the electrolytic production of magnesium metal.
The invention has the advantage of offering a simple, cost-effective way to produce MgCl2 for use in the production of Mg metal. As the production of MgCl2 usually constitutes a large share of the cost of manufacturing Mg metal, the process of the invention thus has the further important advantage of lowering the overall cost of producing Mg metal. This, in turn, has the advantage of making Mg a more viable alternative in automotive and other applications as a strong, lightweight candidate material.